A viscous coupling is composed of a housing, a hub and several tens of thin annular iron plates attached to each of the housing and the hub, with a highly viscous working fluid being confined in the coupling. When a difference in rotational speed is created between the plate assemblies on the hub and housing sides, a shear force is exerted on the two plate assemblies and the torque generated on account of the viscosity of the working fluid is either transmitted or used to control the difference in rotational speed. The construction of a typical viscous coupling is described in Japanese Patent Publication No. 48779/83.
As described above, a viscous coupling depends on the viscous drag of the working fluid for its action and if it is to be installed in an automobile, it is required that the coupling be of the smallest possible size and yet be capable of generating the necessary torque. Therefore, working fluids of comparatively high viscosity have been employed with viscous couplings. Other requirements for working fluids used in viscous couplings are a small temperature dependency of the viscosity and a high stability at elevated temperatures. While silicone fluids have been conventionally used as working fluids, the most common is a dimethylpolysiloxane fluid having a viscosity of 5,000 to 500,000 centistokes at 25.degree. C. However, this fluid has a potential to become very hot due to heat generation from shearing action on the fluid or the friction between plate assemblies and it often happens that during prolonged use, the fluid's viscosity increases until it eventually gels. If the working fluid in a viscous coupling undergoes a significant change in its viscosity or if it gels, the initial setting for the performance of the viscous coupling is no longer applicable. This has been the problem with conventional working fluids for viscous couplings that requires immediate solution.
In any attempt to improve the heat resistance of polyorganosiloxane, the addition of various compounds has been examined. Heat resistance improving agents so far proposed include: amines such as phenothiazine, diphenylamine, phenyl-.alpha.-naphthylamine, N-phenyl-N'-isopropyl-p-phenylenediamine, and N,N'-di-.beta.-naphthyl-p-phenylenediamine; phenols such as 2,6-di-t-butylphenol, styrenated phenol, 4,4'-thiobis(6-t-butyl-m-cresol), and 4,4'-methylenebis(2,6-di-t-butylphenol); salts of octylic acid with metals such as iron, cerium and zirconium, organoselenium compounds, ferrocene, and siloxane compounds having improved miscibility with polyorganosiloxane such as ferro-siloxane, zirconium-siloxane (Japanese Patent Publication No. 14700/81), cerium-siloxane (Japanese Patent Publication Nos. 24377/76 and 980/78), siloxane having an aromatic amino group (Japanese Patent Publication Nos. 18457/80 and 10535/85), and zirconium-cerium-siloxane (Japanese Patent Application (OPI) No. 185597/86 (the term "OPI" as used herein means a "published unexamined Japanese patent application")).
The conventional method of improving the heat resistance of polyorganosiloxane through addition of heat-resistance improving agents proved to be effective in static laboratory-scale heat stability tests, in which a sample in a beaker was left to stand in a thermostatic chamber at 200 to 250.degree. C. and its heat resistance was evaluated in terms of its tendency to increase in viscosity or the time required for it to gel. However, when tested or actual viscous couplings, the modifiers were incapable of preventing the increase in viscosity or gelation of the working fluid in which the modifiers were incorporated.